1. Field of the Invention
The present invention is directed to a valve timing control for an engine, and more particularly to an improved valve timing control for an engine that includes a variable valve timing mechanism.
2. Description of Related Art
Engines, for example, but without limitation, such as those used in marine drives including outboard motors, drive propulsion devices. The propulsion device typically is a propeller and is submerged when an associated watercraft rests on a body of water. The engine can be either a two-cycle engine or a four-cycle engine. Recently, however, many outboard motors have been constructed with four-cycle engines which provide better emission control.
Typically, a four-cycle engine includes one or more intake valves and exhaust valves moving between an open position and a closed position. One or more camshafts can be provided to actuate the valves in a timed manner. With the intake valves opened, air is introduced into combustion chambers of the engine through the intake ports. With the exhaust valves opened, exhaust gases are discharged from the combustion chambers through the exhaust ports.
The engine can include a hydraulically operated variable valve timing (VVT) mechanism that can change opening and closing timing of the respective valves by changing an angular position of the camshaft (or angular positions of the camshafts). A control device such as, for example, an electronic control unit (ECU), is used to control the VVT mechanism under various control strategies. For example, the ECU controls the VVT mechanism to set the valve timing at a fully advanced position for relatively higher engine speeds to ensure high charging efficiency and high performance of the engine. Additionally, the ECU sets the valve timing at a fully retarded position for relatively low engine speeds to ensure high combustion efficiency, fuel economy and good emission control. At other engine speeds, the ECU controls the VVT mechanism to set the valve timing at a position between the fully advanced position and the fully retarded position in response to a running condition of the engine.